Processes for making alpha alumina are known in the prior art. Alpha alumina is usually prepared by calcining aluminum hydroxides, aluminum sulfate, ammonium alum or organic compounds of aluminum at temperatures above 1200.degree. C. The temperature and time of calcination are selected to obtain complete conversion to alpha alumina and elimination of so-called "transition" alumina forms. The calcined material is then milled to fine particle size. Sintered articles of alumina are made by pressing or slip casting the ground powder to required shape and then firing the "green" shape to temperatures of about 1400.degree.-1800.degree. C. to obtain the final product, a dense sintered ceramic article.
Properties of the final product are influenced by two important characteristics of the alpha alumina. These characteristics are (a) size of "ultimate crystallites" of the alpha alumina and (b) chemical purity of the alpha alumina. As used herein, the term "crystallite" refers to that portion of an alpha alumina crystal whose molecules form a perfect lattice without strains or other imperfections. Both density and mechanical strength of the ceramic product after sintering are increased by reducing the crystallite size and by increasing chemical purity of the alpha alumina.
Processes for producing alpha alumina are known in the prior art. However, each of the prior art processes suffers from one or more serious disadvantages making it less than entirely suitable for its intended purpose.
The present invention represents an improvement over the method disclosed in Newsome U.S. Pat. No. 2,642,337 issued June 16, 1953. Newsome's method comprises heating alumina hydrate in contact with steam at a pressure of about 350-2500 pounds per square inch and a temperature of about 400.degree.-550.degree. C., thereby converting the hydrate to alpha alumina. It is preferable to agitate particles of the alumina during the process in order to secure uniform heating and contact between the particles and steam.
The invention claimed and described herein improves upon Newsome's method by heating alumina particles under pressure in a bed fluidized by acidic steam, thereby significantly reducing average crystallite size and average particle size of the alpha alumina product.
Esper et al U.S. Pat. No. 4,019,914 discloses a process for manufacturing alpha alumina said to have excellent sintering activity. The process requires first finely grinding a mixture of alumina and a "calcination auxiliary agent" which may be Fe.sub.2 O.sub.3 or Cr.sub.2 O.sub.3. The mixture is calcined at 1120.degree.-1350.degree. C., cooled, and then further ground to produce particles having a "grain size" of not more than 0.2 micron.
Matyasi et al U.S. Pat. No. 4,477,427 claims a process of forming alkali-poor alpha alumina. The process requires activating aluminum hydroxide at 500.degree.-600.degree. C., washing the activated alumina with an alkali-free solution containing calcium ions, and then calcining at about 1200.degree.-1350.degree. C. in the presence of a mineralizing mixture comprising halides and boron compounds. The latter mixture preferably contains boric acid and aluminum fluoride and may also contain calcium fluoride.
British Patent No. 802,731 discloses a process for producing alpha alumina having low sodium content. Alumina powder is heated to a temperature of about 1300.degree.-1700.degree. C. until a substantial part of the sodium content is vaporized. The powder is then cooled, and finally washed with hot hydrochloric acid and hot water until sodium oxide content is further reduced. Grain size of the powder is said to be 1-10 microns. The patent fails to suggest treating alumina powder with an aqueous acid solution or with steam having the pH and temperature conditions claimed herein.
None of the prior art references cited above discloses or suggests treating alumina powder with acidified steam at a temperature of 380.degree. C. or higher.
It is a principal objective of the present invention to provide a process for producing alpha alumina having very small ultimate crystallite size.
Other objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification and claims.